Vibrating screen machines for use in the mineral processing industries are commonly used to separate minerals such as coal or ores by size, usually after crushing. The apparatus generally comprises a screening deck having RHS or boxed I-beam screen support members, the screen support members being spaced apart to support the ends of modular screen inserts rigidly between side plates that are further interconnected by other cross members. The screen deck may be flat or may be curved to form a so-called banana screen. The screen apparatus is vibrated by exciter assemblies generally mounted on a heavy cross beam located at the top of the apparatus.
Australian patent specification AU-B-20043/95 describes a vibrational exciter for a screen machine comprising a pair of eccentric masses mounted for counter rotation on respective shafts, a pair of corresponding drive means disposed respectively to effect rotation of the eccentric masses and synchronization means adapted to establish a predetermined rotational velocity and phase relationship between the eccentric masses. The synchronization means allow effectively independent rotation thereof when the steady state of predetermined velocity and phase relationship is achieved. In practice this was achieved by the provision of 4 mm (1.43° of rotation) of lash. Since the gears do not transmit power in this steady state operation, there is a significant reduction in noise.
It has been determined that the direction of excitation should pass through the centre of gravity of the machine in use. However, as the screening apparatus wears, or panels are changed for a different brand, or the machine is loaded with a mass of material and progressively shifts this mass, the centre of gravity moves relative to the direction of vibration.